1. Field of the Invention
The present invention relates to a variable ratio rack and pinion steering gear for road vehicles.
2. Description of the Prior Art
A typical form of variable ratio rack and pinion steering gear is disclosed in the Japanese Patent Publication No. 52-29049 and wherein the pinion is formed as a standard helical pinion having identical involute teeth whilst the rack is formed as a variably contoured rack having teeth of variable shape and pitch. The rack teeth are adapted to be generated by using a cutter having blades of the same tooth form as that of the pinion, i.e., the rack teeth are adapted to be generated by using the profile of the pinion teeth.
The rack and pinion steering gear of the above-described type encounters a difficulty of obtaining adequate strength in the rack, particularly when a relatively wide variation of gear ratio is required. This is due to the fact that the rack teeth have, at the region for providing a relatively small gear ratio, a tendency to have a large pressure angle and therefore to be sharp-edged at the tops and also have, at the region for a relatively small gear ratio, a tendency to have undercuts at the roots. In the case where this tendency is pronounced, a considerably wide variation in strength of the rack teeth and therefore a considerable reduction in steering force transfer efficiency will inevitably result.